Hydrostatic transmissions use a hydraulic fluid to transmit power from a power source (for example, an internal combustion engine) to a power output (for example, a final drive or a plurality of wheels). Hydrostatic transmissions are typically used in agricultural tractors and other off-highway equipment, for example, forklifts, excavators, earth moving machines, and other vehicles,
The major benefits of hydrostatic transmissions are a large range of continuously variable speed, a precise control of traction effort and speed, and high maneuverability. Each of these benefits is directly related to vehicle productivity. Other advantages include high power capability in a compact size, a fast response related to low inertia, maintaining a controlled speed regardless of load, high traction force at a low engine speed, flexibility in packaging, dynamic braking, and simplicity in reversing vehicle direction. Compared to traditional solutions, such as a hydrodynamic transmission with a torque converter, hydrostatic transmissions can provide improved performance. As a non-limiting example, a wheel loader application may require high maneuverability and a wide torque and speed conversion range.
Hydrostatic transmissions are not without their drawbacks, however. Hydrostatic transmissions tend to have a lower overall efficiency, increased maintenance costs, and increased initial investment cost compared to conventional gear transmissions. As a result, design considerations for a given application in a hydrostatic transmission are very important. As a non-limiting example, a hydrostatic transmission design can focus on one or more particular operating modes, such as low speed driving to provide maximum tractive effort, variable speed operation, or maximum speed operation. Focusing a design on an operating mode, will increase an overall efficiency of the transmission and proper sizing of transmission components will result in a more cost-effective solution.
A hydrostatic driveline can be divided into many standard categories based on the characteristics of the hydraulic pump and the hydraulic motor. The hydrostatic driveline can include a fixed displacement pump or a variable displacement pump and a fixed displacement motor or a variable displacement motor. A common combination amongst hydrostatic drivelines is a driveline configures with a variable displacement pump and a fixed displacement motor. In this combination, an output speed is controlled by varying a displacement of the pump.
To increase versatility of a hydrostatic driveline, such as including a high output capacity and a wide velocity of operational ranges, many alternative concepts of hydrostatic drivelines have been developed to meet such demands. One of the simplest and most common solutions is to use the hydrostatic transmission with a mechanical gearbox connected in series, and is shown in FIG. 1. A hydrostatic driveline 1000 includes a power source 1002 in driving engagement with a hydrostatic pump 1004. The hydrostatic pump 1004 is in fluid communication with a to hydrostatic motor 1006. The hydrostatic motor 1006 is in driving engagement with a transmission 1008, which is in driving engagement with a vehicle output 1010. As shown in FIG. 1, the hydrostatic pump 1004 and the hydrostatic motor 1006 may be bypassed through the use of a torque converter 1012.
It would be advantageous to develop a hydrostatic driveline that includes a direct drive capability, which offers the benefits of increased efficiency at a high speed operating mode while maintaining the benefits of a hydrostatic drive at a low speed operating mode.